This invention relates to a full-additive type printed circuit board forming a circuit by electroless plating in which insulation resistance of the board after moisture absorption will not be lowered even if the catalyst for electroless plating remains between the circuits, a process for producing such printed circuit board and a resist ink used therefor.
For the production of printed circuit board, a method is known in which an electroless plating reactive catalyst is formed on the portion of the insulating board surface where a circuit is to be formed while a resist for electroless plating is provided on the insulating board excepting the portion where a circuit is to be formed, and a circuit is formed on said portion by electroless plating.
FIG. 1 is a schematic sectional view of a printed circuit board having throughholes on both sides obtained according to an example of this production method. In FIG. 1, reference numeral 1 indicates an insulating board, which may have adhesive layers on both sides, of which at least one surface has a catalyst for electroless plating indicated by numeral 2. Numeral 3 refers to a resist layer for electroless plating provided on the insulating board surface excepting the part where a circuit is to be formed. Numeral 4 denotes a circuit formed by electroless plating, and 5 indicates a conductive through-hole. This method, however, had the problem that insulation resistance between the circuits drops excessively when the board is subjected to a moisture absorbing treatment, because the catalyst for electroless plating remains under the resist for electroless plating between the circuits.
It is considered that such drop of insulation resistance is caused as the catalyst for electroless plating, which is generally composed of palladium or like material, is ionized, for instance under a voltage of 500 V, when moisture penetrates into the board in a moisture absorbing treatment.
As a solution to this problem, U.S. Pat. No. 4,430,154, proposes to remove the catalyst remaining on the insulating board surface between the circuits, along with the adhesive layer, by using an alkaline permanganate solution or chromate solution. According to this method, lowering of the insulation resistance after moisture absorption is prevented. However, since a permanent mask is used as the resist for electroless plating in said production process, it is very difficult and troublesome to remove the adhesive layer under the resist, and also an extra step must be added to the process.